cli/context/store: Names(): fix panic when called with nil-interface

Before this, it would panic when a nil-interface was passed.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
(cherry picked from commit e4dd8b1898)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/context/store/store.go

@@ -124,6 +124,9 @@ func (s *ContextStore) List() ([]Metadata, error) {
 
 // Names return Metadata names for a Lister
 func Names(s Lister) ([]string, error) {
+	if s == nil {
+		return nil, errors.New("nil lister")
+	}
 	list, err := s.List()
 	if err != nil {
 		return nil, err

cli/context/store/store_test.go

@@ -260,3 +260,9 @@ func TestCorruptMetadata(t *testing.T) {
 	_, err = s.GetMetadata("source")
 	assert.ErrorContains(t, err, fmt.Sprintf("parsing %s: unexpected end of JSON input", contextFile))
 }
+
+func TestNames(t *testing.T) {
+	names, err := Names(nil)
+	assert.Check(t, is.Error(err, "nil lister"))
+	assert.Check(t, is.Len(names, 0))
+}